The burning of coal provides a major source of electric power in the United States. It has increasingly become apparent that coal-burning power plants are a major source of the pollutants, including SO.sub.2 and NO.sub.x, which are responsible for damage to fish and plant life in the northeastern part of the country and in Canada due to the phenomenon now known as "acid rain". The acid rain problem is complex and the steps necessary to control the problem are not easy to accomplish. For example, coal is the most abundant source of fossil fuel and will be available long after the earth's available petroleum supply is exhausted. Unfortunately, most of the coal supplies in the Eastern and Midwestern United States are high in sulfur, and substitution of lower-sulfur Western coals therefore is not only expensive because of transportation cost but can cause further distress in the already economically deprived coal-mining areas.
It is accordingly desirable that economic means be found whereby available high-sulfur coals could be utilized without further contributing to the atmospheric pollution problem.
It is known also that numerous types of coal-burning apparatus are available for large-scale coal combustion for purposes such as steam generation. Thus, the cyclone burner was developed in the 1940's particularly for the purpose of burning an Illinois coal which has a high ash content and a low ash-fusion temperature. A paper entitled "Operating Experiences With Cyclone-Fired Steam Generators" by V. L. Stone and I. L. Wade which appeared in Mechanical Engineering, Vol. 74, 1952 at pages 359 to 368 describes operation of a power plant using cyclone burners. The book Low-Rank Coal Technology; Lignite and Subbituminous by Gronhovd and Sondreal of the Grand Forks Energy Technology Center and Kotowski and Wiltsee of the Energy Resources Company, Inc. published by the Noyes Data Corporation in 1982 provides further information. Gronhovd et al. point out that the cyclone furnace promotes complete combustion of coal in a high temperature, turbulent slagging environment and is applicable to all ranks of coal. Cyclone firing is considered to reduce the fly ash content of the flue gas. Heat release rates are extremely high, hence local temperatures are high and are sufficient to fuse the ash from most coals on the refractory walls of the cyclone.
Gronhovd et al. point out that the cyclone furnace is a water-cooled, refractory-lined cylinder. Crushed or pulverized coal and primary air are fed at the burner end of the furnace and secondary air is fed into the cylinder tangentially, thus creating a whirling or cyclonic motion to gases within the cylinder. Coal particles are entrained in the high velocity stream and thrown against the furnace wall by centrifugal force where they are held in the molten slag layer. The high-velocity tangential stream of secondary air supplies combustion oxygen to the coal particles. Molten slag drains to the bottom of the furnace from which it is removed. The cyclone furnace is thus a slagging type of coal burner.
Reference may also be made to U.S. Pat. No. 2,745,732 which describes use of a cyclone type furnace under strongly reducing conditions to burn coal and to reduce and/or melt iron ores fed into the furnace.
Proposals have been known, as for example, U.S. Pat. No. 4,095,960 for gasifying high sulfur coal in a fluid bed or the like in the presence of lime (CaO) to fix sulfur as CaS.